1. Technical Field
This disclosure generally relates to an image forming apparatus that forms an image by a recording head which ejects ink droplets of recording liquid.
2. Description of the Related Art
A conventional polarization control device such as a polarizer or a wavelength plate makes propagation characteristics and absorption characteristics anisotropic relative to two orthogonal directions. The device thereby transmits one of two components of an incident light in two polarization directions, or modulates a phase of the incident light to change a polarized state of the incident light from a linear polarized state to a circular polarized state. The device of this type is used, for example, to turn on or off pixels on a liquid crystal panel or an organic electroluminescence (EL) display. In addition, the device is used for an optical measurement technique such as an ellipsometry (polarization analysis), and in any one of various optical apparatuses and measurement apparatuses such as a laser interferometer and an optical shutter. Demand for the polarization control device rises particularly for an image projecting apparatus such as a liquid crystal projector.
The polarizer is a device that transforms a naturally polarized light into a linearly polarized light, and that transmits only one of two orthogonal polarized components of the incident light while shielding the other polarized component by absorption (or reflection and scattering). Most of the polarizers currently available and particularly used in liquid crystal panels are each configured so that a dichroic material such as iodine or an organic dye is colored and attached onto a substrate film consisting of polyvinyl alcohol or the like. In addition, the resultant film is drawn and oriented in an advanced fashion, thereby expressing absorption dichroism.
On the other hand, a retardation plate (or phase shifter) such as a half-wave plate or a quarter-wave plate consists of a birefringent optical crystal, and modulates a polarized state by a difference in refractive index between an ordinary ray and an extraordinary ray. The retardation plate for which an optical path difference between the ordinary ray and the extraordinary ray is a half of a wavelength of the incident light is the half-wave plate. That for which the optical difference is a quarter of the wavelength thereof is the quarter-wave plate. As the material that exhibits the birefringence, calcite or crystal is used.
However, the conventional polarization control device making use of absorption has the following disadvantages. The device is apt to be influenced by heat, so that a transparency of the device is deteriorated and the device is burnt. As a result, an amount of irradiation light cannot be increased. In addition, utilization temperature conditions for the device are strict. For this reason, if the device is used in the liquid crystal projector or the like, it is necessary to provide a cold air blasting mechanism in the projector, whereby it is difficult to make the projector small in size. Furthermore, because of adhesion of dust to the device, an image-quality defect occurs.
The conventional polarization control device using anisotropy of the refractive index has the following disadvantages. Optical crystal materials exhibiting birefringence are limited in number and available wavelength ranges are limited. By bonding the optical crystal materials, a film thickness, i.e., an optical path difference is adjusted, and the polarized state is controlled. Therefore, the polarization control device is largely dependent on the optical crystal materials and low in flexibility of polarization control. In addition, it is difficult to make the polarizer small in size and thin per se.
To overcome the problems, various conventional techniques are disclosed including a polarizer that blocks one of the two polarized components of the incident light. Therefore, with a configuration of an optical system constituted only by this polarizer, optical energy of the incident light is lost by at least 50%. In addition, the wire-grid polarizer cannot ensure a sufficient extinction ratio.
A mechanism that can control a plane of polarization according to a nanoscale metal pattern is also disclosed. Similarly, since this mechanism is based on an operation principle using the anisotropy of absorption and scattering, high optical utilization efficiency cannot be attained.
Another conventional technology has a function corresponding to a phase plate and transforming a linearly polarized light into an elliptically polarized light using chirality of a nanoscale metal pattern. However, this function cannot ensure a high ellipticity. It is, therefore, difficult to realize a practical half-wave plate or quarter-wave plate using this function. In addition, a structure of the function is complicated, and it is difficult to accurately manufacture a structure having uniform chirality because of low machining accuracy. The shape irregularity makes it impossible to obtain desired polarization control characteristics.
Furthermore, the polarization control devices according to the conventional techniques are all passive optical devices having optical response characteristic of the respective devices. If a function to be realized is more complicated, the number of optical devices increases. This imposes restrictions on making an optical system small in size and light in weight.